DISTRIBUTION 409 



resembles closely the parent. These metamorphoses at 

 times give us clues as to the past history of the group. 

 Thus the larvae of Echinoderms (p. 273) and the tadpoles 

 of the Anura (p. 337) point to the fact that the first group 

 has descended from markedly bilateral ancestors, and that 

 the radiate condition of the adult has been secondarily 

 acquired; while the history of the frog is evidence that 

 these amphibians have sprung from tailed water-breathing 

 ancestors. In the insects, on the other hand, the larval 

 and pupal stages have far less significance, but have 

 apparently been introduced into the history 7 the better to 

 adapt these forms to the various conditions of their exist- 

 ence (p. 242). 



GEOGRAPHICAL DISTRIBUTION. 



The most superficial observation shows that the differ- 

 ent regions of the earth are inhabited by different animals, 

 and the same, though not so evident, is true of the sea. 

 The animals found in a given region constitute its fauna, 

 and the study of these faunae and the causes underlying 

 them is one of the most interesting branches of zoology. 



From detailed studies of the distribution of the higher 

 vertebrates most zoologists have come to recognize six 

 primary land regions : (1) ThePalsearctic, including Europe 

 and the southern shore of the Mediterranean, and Asia 

 south to the Himalayas; (2) the Ethiopian, including 

 Africa south of the Sahara; (3) the Oriental, including 

 southern Asia and the islands as far east as Celebes, (4) 

 the Nearctic, embracing North America south to about 

 the Mexican boundary; (5) the Neotropical, consisting 

 of the rest of the American continent; and (6) the Aus- 

 tralian, composed of Australia, the Islands of the Pacific, 



